1. Field of the Invention
This invention relates to improved coating compositions. More particularly, the present invention relates to rheological additives for coating compositions. Even more particularly, the present invention relates to thixotropic additives which control sag, levelling and pigment suspension in a coating composition. The rheological additives of the present invention are copolymers having a backbone of recurring .alpha.-olefin and carboxylic acid monomeric units, having grafted thereto one or more polymeric grafts with the following recurring monomeric units: EQU --NHC(O)RC(O)NHR.sub.1 and--NH--R--C(O)--
or a combination thereof, wherein R is an alkylene group of at least about 2 carbon atoms, and R.sub.1 is R or aryl.
2. Description of Related Art
In the formulation of coatings, it is well known that rheological modifiers may be added to control the flow properties of the final product for a particular application. A large number of rheological additives are available to control properties of a coating formulation during storage, transportation, processing, application, and post-application to a particular surface. These additives include modified and unmodified organic clays, and a large variety of organic compounds, as set forth, for example, in Rheology Handbook, Rheox, Inc., Hightstown, N.J. 1991.
The control of sag, levelling and pigment suspension in a coating formulation requires additives which maintain viscosity of the coating at a variety of shear rates. This is difficult, since control of these properties involves opposing phenomena. For example, total freedom from sag requires a coating with extremely high viscosities during application; however, the low flow rate of such a material means that the coating may not flow out completely to form a smooth film. The development of an enveloping rheological structure to control pigment suspension during storage may result in coating which exhibits levelling problems during application.
To maintain viscosity under these diverse conditions, the prior art has suggested incorporation of additives to control pigment suspension, such as surfactants and thickeners, along with additional additives intended to control levelling and sag, such as organic clays. However, production of a coating with such multiple additives is a difficult and complex procedure, sometimes requiring separate processing conditions for each rheological additive. A correct balance of properties must also be achieved among the various additives. In the prior art, even processing parameters of single rheological additives have proven quite difficult to control. If these precise formulation requirements are not employed, coatings with inconsistent properties may be produced. These precise formulations are also difficult to manufacture on a large scale, and require maintenance of a large variety of additive products.
The rheological modifier utilized in the improved coating composition of the present invention performs the functions of both an anti-sag additive and an anti-settling additive. In addition, the process of producing the coating composition of the present invention does not require the multiple formulation steps or precise parameter control necessary to compound the coatings of the prior art.